In one type of reciprocating piston compressor, the piston is connected through a wrist pin to a connecting rod. The piston is restrained to strictly linear movement as it reciprocates in a cylinder. In this type of compressor, adequate oil lubrication is critical to the life of the compressor.
In a second type of reciprocating piston compressor, the piston is formed integral with the connecting rod. As the lower end of the connecting rod is rotated by an eccentric on a crank shaft, the piston reciprocates and also rocks relative to the cylinder axis. Sufficient clearance must be provided between the cylinder wall and the piston to permit the required rocking motion. A resilient piston ring or seal is used to maintain a sliding seal between the piston and the cylinder walls as the piston reciprocates and rocks. The seal does not require oil lubrication and, consequently, this type of compressor is sometimes referred to as an oilless compressor. Normally, the maintenance free operating life of an oilless compressor is determined by the life of the piston seal. In many prior art oilless compressors, the piston seal requires replacement after only a few hundred hours of operation, and sometimes after as few as 50 hours of operation.
In a conventional oilless compressor, the axis of the cylinder intersects the axis of the crank shaft. As a consequence, the piston will be perpendicular to the cylinder axis at both top dead center and bottom dead center. At 90 degrees of crank shaft rotation after top dead center and at 90 degrees of crank shaft rotation after bottom dead center, the piston will have equal and maximum angles of tilt relative to the cylinder axis. Accordingly, the piston will rock through the same angle during the compression stroke when it is under load as during the intake stroke when load is at a minimum.